A rain sensor lights a light emitting element such as a light emitting diode (LED) at a regular frequency, irradiates the light from the light emitting element to a windshield (front glass), receives a reflected light by a light receiving element such as a photodiode (PD), takes in an output signal (pulse signal) of the PD in a microcomputer and finally measures a rainfall level by detecting raindrops or an adhesion amount of raindrops.
FIG. 1 shows a signal detecting mechanism of the rain sensor. A light 11 from an LED 10 goes through lenses 1 2 and a prism 14 and is reflected by the surface of a front glass 16, goes through a prism 14 and a lens 18 and enters a PD 20.
A signal detecting circuit for detecting a pulse signal outputted by the PD 20 is, as shown in FIG. 2, comprised by an analog circuit and a microcomputer. The analog circuit 22 is comprised by a current—voltage (I-V) converter circuit 24, a band-pass filter circuit/amplifier circuit 26 and a peak hold circuit 30. The pulse signal obtained from the PD 20 is converted by the I-V converter circuit 24 from change of a current value to a change of a voltage value. A noise component is eliminated and amplified by the band-pass filter circuit/amplifier circuit 26 and finally, a peak of the amplified pulse signal is held by the peak hold circuit 30. The held peak value is sent to a microcomputer 32.
The microcomputer 32 is provided with an A/D converter 34 and processes a digital value obtained from the A/D converter with software to obtain raindrop information. The rainfall level is determined from the raindrop information.
With the rain sensor with the structure shown in FIG. 1, the light received by the PD 20 includes not only the reflected light from the LED 10 but also, as shown in FIG. 3, light from the outside environment, that is, an outside light 13, and about as much as 90% of the light received by the PD is this outside light component. The components of the outside light include those with approximately constant light intensity and those with fluctuating light intensity. Hereinafter, those with constant light intensity will be referred to as a constant outside-light component and those with fluctuating light intensity as a fluctuating outside-light component.
FIG. 4 shows an output waveform of the I-V conversion circuit 24 with the constant outside-light component. It is understood that the PD pulse signal is overlapped on the constant outside light component. A bias voltage level is a voltage level when the PD 20 is operated by a reverse bias.
FIG. 5 shows an output waveform of the I-V conversion circuit 24 with the fluctuating outside-light component. It is understood that the PD pulse signal is overlapped on the fluctuating outside-light component.
If the constant outside-light component is included in the outside light 13, as shown in FIG. 6, the voltage of the pulse signal outputted by the PD 20 is raised by the outside light component. When it exceeds a saturation voltage of an operational amplifier of the I-V conversion circuit 24, the signal component is crushed and a correct signal level is not inputted into the A/D converter 34. Therefore, such a problem is caused that a correct rainfall level cannot be determined.
On the other hand, if the fluctuating outside-light component is included in the outside light, and its frequency band comes into a pass band of the band-pass filter circuit/amplifier circuit 26, a high frequency component of the fluctuating outside light passes through the band-pass filter circuit/amplifier circuit 26 and as a result, the S/N ratio of an output value of the peak hold circuit 30 is deteriorated. The microcomputer 32 cannot determine the correct rainfall level, which is a problem.
There are the following two methods to reduce the above outside light component including the constant outside-light component and the fluctuating outside-light component.
The first method is a method using a visible-light cutting (absorbing) prism as the prism 14 shown in FIG. 1. With this method, however, the reflected light from the LED is also attenuated by the visible-light cutting (absorbing) prism, and the signal received by the PD is lowered, which is a problem.
The second method for reducing the outside-light component is a method for cutting the outside-light component by increasing the number of reflections in the prism 14 to prevent the outside light from directly entering the light receiving element. With this method, however, the size of the prism is increased and hence, the size of the rain sensor itself is increased, which is a problem.